In the electrophotographic copying machine or printer, a fixing device for fixing a toner image on a recording material by heating the recording material while feeding the recording material on which the toner image which has not been fixed is carried is mounted. As this fixing device, a fixing device of an electromagnetic induction heating type has been known. The fixing device of this type has advantages such that a temperature rise of the cylindrical film (rotatable heating member) for heating the recording material is quick and that also electric power consumption is low.
In Japanese Laid-Open Patent Application (JP-A) 2014-26267, a fixing device of an electromagnetic induction heating type in which an exciting coil and a magnetic core are provided inside of a cylindrical rotatable heating member and an alternating magnetic field is generated with respect to an axial direction of the rotatable heating member, and then the rotatable heating member is heated by a circumferential current generating around an electroconductive layer of the rotatable heating member with respect to a circumferential direction has been disclosed. In JP-A 2014-26267, as a material of the electroconductive layer as a heat generating layer of the rotatable heating member, metal is employed. The metal is low in volume resistivity, and therefore even at a voltage value of a commercial power source level, the circumferential current sufficiently flows, so that also temperature rise is quick.
On the other hand, when a resin film of polyimide or the like can be employed as the rotatable heating member, it is desirable from viewpoints of a cost and flexibility. However, even when electroconductivity is imparted to the resin film by adding an electroconductive agent such as carbon black, there was a limit that the volume resistivity of the resin film is lowered to about 1×10−4 Ωm. For that reason, at the voltage of the commercial power source level, the circumferential current merely flows a little, and therefore a temperature rising speed is slow, so that there is a problem that it is difficult to employ the resin film as the rotatable heating member.